Never seen sigil create any style other than SGC-# (and that is inline in the head of the document)

Sure, when I had Tidy turned on, I used to enjoy Sigil's habit of creating its own styles and sticking them at the top of the relevent xhtml document so it would have precedence over the entire-ePub-stylesheet. In fact I used to use it as sort of a "suggestion list" of styles that maybe I should create and put in the main stylesheet instead of inserting inline HTML ... sort gently coercing me to get into good CSS habits. I finally got tired of trying to find the SGC number to figure out what the Sigil-created style did, plus I figured I had learned my lesson well enough to do without Sigil's help. But this problem is totally different -- a Sigil-invented style that seems to exist nowhere and do nothing, except that it deleted the <i> and </i> tags that it replaced. I finally just defined a style with the same name in the main style sheet to italicize the text it enclosed, which I guess solved my problem for now. But I'm wondering: what gives? I never had the problem with previous versions of Sigil. Actually, out of curiosity, I created an undefined do-nothing style (<span class="xxx"> and control-clicked on it to see where it would take me for the definition ... and it took me to the top line of the main stylesheet, just like the Sigil-invented do-nothing style. But the Sigil-invented do-nothing style didn't simply "do nothing"; it also removed the <i> and </i> do-SOMETHING tags that I wanted and didn't replace it with a style that accomplished the same thing. So it actually did damage. Incidentally, and totally changing the subject and probably violating all kinds of forum rules, have you noticed that Sigil 0.6 often has difficulty (= can't do it) saving an ePub. I did report the error and included the full error message. If you're having that problem but don't want to give up on 0.6 yet, it will usually work to "save a copy" ... but then there's the whole renaming rigmarole to go through.